Manipulation and active control of an incident intense radiation by metallic objects have been demonstrated using localization of plasmons in subwavelength dimensions ([1], [2]). Providing real-time spectral response control plasmonics has emerged as a promising technology for tailoring fast, efficient, and tightly integrated nanodevices for photonic applications ([1], [2], [3]). Rising demands for miniaturized and multifunctional all-optical devices requires advances in integration of the next generation of photonic circuits. Among several potential applications of plasmonics technology, the biomedical applications still need to be improved for quick infection diagnosis and real-time pharmacology purposes ([4], [5], [6]). Plasmonic metasurfaces with exotic electromagnetic response have been relatively better developed for advanced label-free detection in biosensing applications in the spectral ranges from near-infrared wavelengths ([7]) to the terahertz (THz) ([8], [9]) and microwave ([10], [11]) frequencies. It is well-accepted that THz frequencies are highly compatible with human tissues due to absence of ionization hazard because of their low energy of the incident radiation (in the range of a few meV) ([8], [9], [12], [13]). This spectacular advantage of THz plasmonic structures accompanied with cost-effective and easy microfabrication techniques (photolithography) have stimulated researchers to exploit THz plasmonics for immunosensing applications ([14], [15]).
As a promising technique, THz spectroscopy allows for non-invasive, non-contact, non-destructive, and label-free biomarker detection and therefore attracts growing interest for biomedical and clinical applications ([15], [16], [17], [18], [19]). It is shown that electromagnetic field enhancement and confinement by metallic THz components facilitate detection of targeted bio-agents such as specific proteins, antibodies, and etc., ([9], [16], [17], [19], [20], [21]). Despite of such a unique potential, the selectivity and sensitivity of THz metasurfaces for immunosensing sensing applications have not been analyzed comprehensively due to mismatch between resonance frequency of nanoscale bio-targets and metasurfaces. This is because of non-responsivity of micro- and nano-organisms with the size of approximately ˜λ/100, causing to be almost transparent to the incident radiation, therefore, reflect poor scattering cross-section ([22]). This challenge in THz metamaterials can be circumvented using two approaches: 1) introducing nanosize particles (e.g. nanospheres) ([9], [23]) on the micro-scale plasmonic chips to trap and bind biological objects and monitor their effect on the spectral response, and 2) excitation of ultrasharp antisymmetric resonances (with high-Q-factors) to show super-sensitivity to the small environmental variations. Here, by using the high-Q advantage of magnetic dipolar toroidal moment, the behavior of this mode is analyzed for the presence of Zika-virus (ZIKV) envelope protein and its specific antibody. Zika is a new medical threat across the world as an infectious disease which causes serious health disorders, possibly leading to death ([24], [25]). Various methods have been introduced and conducted for practical detection of this type of infection such as reverse transcriptase-PCR ([26]), antibody-based methods (e.g. ELISA) ([26]), point-of-care (POC) molecular detection ([27]), and electrochemical biosensing ([28]). Despite of the growing research, most of these applications suffer from high-costs, lack of sensitivity and repeatability, and complex processing. Therefore, providing an all-optical microscale metasurface with an ability to detect picomolar concentrations of ZIKV envelope protein would help us to tailor practical, easy to fabricate, and accurate detection mechanism with high reliability.
The exquisite properties of plasmonic structures with symmetric and antisymmetric geometries allow for achieving spectral line shapes as pronounced resonant modes in the optical band reaching to the far-infrared region (FIR) ([29], [30]). The response of these resonant modes to the polarization variations of incident radiation and physical changes in their surrounding medium have triggered development of fast switches and high-precision sensors ([31], [32], [33]). Recently, successful examples of plasmonic line shapes have been introduced for practical photoswitching applications based on Fano-resonant metallic assemblies ([34]), asymmetric line shapes in Mach-Zehnder interferometers ([35]), electromagnetically induced transparency (EIT) ([33]), and Lorentzian spectral line shapes ([36]). The corresponding linewidth of these resonant modes is defined by full-width at half-maximum (FWHM), known as quality-factor (Q-factor) ([37]). The sharpness and depth of these resonance line shapes play crucial role for developing advanced high-Q integrated devices ([38], [39]). However, sustaining strong resonance coupling between optically excited modes in planar 2D structures is difficult and achieving ultrasharp and substantially high-Q factor line shapes is a serious challenge.
Recent analyses have shown that plasmonic metasurfaces composed of 3D ([40], [41], [42]), and 2D ([43], [44]) unit cells can be tailored to support distinct toroidal multipolar modes which are categorized in different resonant modes family far from the classical electromagnetic modes ([45]). These hardly distinguishable modes can be identified as the magnetic multipoles that are condensed into a single spot corresponding to a unique current density ([43], [44], [45], [46]). In the toroidal moment limit, oscillating radial components of the current density are included in the radiative fields, giving rise to the formation of a unique family of dynamic toroidal multipolar modes ([42], [47]). Toroidal dipolar resonant mode has been observed in 3D structures, however, challenging and complex fabrication processes limit their practical applications ([43], [48], [49], [50], [51]). Therefore, excitation of toroidal multipolar modes in planar 2D structures has received growing attention recently due to their easy fabrication and characterization ([43]-[51]). The primary problem with 2D structures is the inherently weak coupling of magnetic fields at the dielectric spacer of the resonators, which makes detecting the magnetic toroidal modes very difficult. Confinement of the incident magnetic field inside a unit cell into a rotating torus loop is another problem correlating with planar structures.